An information recording/reproducing device and a medium are improved in reliability by making medium managing information usable even when a defective sector exists in the medium managing information area of the medium. The identifier of medium managing information, the information about the position where the medium managing information is recorded which is the information about the position of the sector in use, or the information about the position where a defect exists in the medium managing information area, which is the information about the position of the defective sector, are read from the medium managing area of the medium (3) and all the information is stored in a RAM (8) by using a medium managing information position manager. A reproducer and a recorder are controlled based on the information. Therefore, the medium managing information is correctly recorded and the reliability of the data of the information recording/reproducing device and the medium is improved, because the medium managing information is not recorded in the defective section.